Add instructions for checking out main and making a new branch to HTML/CSS coursework

[40thieves]

Which module(s) and week(s) does this change affect?
Module(s): HTML/CSS
Week(s): 2 & 3

What is the work that needs to be done?

Add instructions to the Cakes Co coursework (CodeYourFuture/Module-HTML-CSS#13) and Form Control coursework (CodeYourFuture/Module-HTML-CSS#6) on how to:

1. Switch to the main branch
2. Create a new branch from main for the relevant week's coursework

Why is this work important to do?

In #223, we discussed how we wanted to introduce Git branching strategies. We concluded that the "feature branch-esque" strategy (i.e. 1 branch per week) is the most "realistic".

Therefore we should add some instructions when trainees will be creating branches for the first time: in week 2 of the HTML/CSS module.

Additional context

Who might need to know about this change?

@CodeYourFuture/global-syllabus

[40thieves]

I've made a proposal to change the tickets here: https://gist.github.com/40thieves/fb2f9de4999e1b5cfa3b5eb1572d4113. @CodeYourFuture/global-syllabus could you please review?

If we're happy with that, I can copy the changes into the tickets. Also as noted in a comment it would be helpful if someone with access to the CYF Youtube account could create "clip" for the relevant piece of the VSCode video.

[SallyMcGrath]

I'm happy with it! And I made the clip, I think

https://youtube.com/clip/Ugkx2OzD279ry4XKFNpeyqJTeBqtrbGJfgRq

[illicitonion]

LGTM!

[40thieves]

Tickets have now been updated. Closing.